1. Field of the Invention
The present invention relates to an anode active material containing silicon as an element and a secondary battery including the anode active material.
2. Description of the Related Art
In recent years, many portable electronic devices such as camcorders, mobile phones, and notebook personal computers have been introduced, and their size and weight have been reduced. Since a battery used as a portable power source for these electronic devices, in particular a secondary battery is important as a key device, research and development to improve the energy density has been actively promoted. Specially, a nonaqueous electrolyte secondary battery (for example, lithium ion secondary battery) provides a higher energy density compared to a lead battery and a nickel cadmium battery as an existing aqueous electrolytic solution secondary battery. Thus, studies of improving such a nonaqueous electrolyte secondary battery have been made in various fields.
In the lithium ion secondary battery, as an anode active material, a carbon material such as non-graphitizable carbon and graphite that shows the relatively high capacity and has the favorable cycle characteristics has been widely used. However, since a higher capacity has been demanded in recent years, the capacity of the carbon material should be further improved.
Against such a background, techniques to retain a high capacity with the use of the carbon material by selecting the carbonized raw material and the forming conditions have been developed as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 8-315825. However, in the case of using such a carbon material, the anode discharge potential is from 0.8 V to 1.0 V both inclusive to lithium, and the battery discharge voltage is lowered when the battery is fabricated. Thus, in this case, it is not possible to expect great improvement in the battery energy density. Further, in this case, there is a disadvantage that the hysteresis is large in the charge and discharge curved line shape, and the energy efficiency in each charge and discharge cycle is low.
Meanwhile, as an anode with the higher capacity than that of the carbon material, researches on an alloy material have been promoted. In such an alloy material, a certain type of metal is electrochemically alloyed with lithium, and the resultant alloy is reversibly generated and decomposed. For example, a high capacity anode using Li—Al alloy or Sn alloy has been developed. In addition, a high capacity anode made of Si alloy has been developed as disclosed in, for example, U.S. Pat. No. 4,950,566.
However, the Li—Al alloy, the Sn alloy, or Si alloy is expanded and shrunk due to charge and discharge, the anode is pulverized every time charge and discharge are repeated, and thus the cycle characteristics are extremely poor.
Thus, as a technique to improve the cycle characteristics, studies on suppressing expansion by alloying tin or silicon have been made. For example, it has been proposed to alloy iron and tin as disclosed in, for example, “Journal of the Electrochemical Society,” 1999, No. 146, p. 414. Further, Mg2Si or the like has been proposed as disclosed in, for example, “Journal of the Electrochemical Society,” 1999, No. 146, p. 4401. Furthermore, for example, Sn•A•X (A represents at least one of transition metals and X represents at least one selected from the group consisting of carbon and the like) in which the ratio Sn/(Sn+A+V) is from 20 atomic % to 80 atomic % both inclusive has been proposed as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2000-311681.